1. Field of the Invention
This invention relates to containers for the storage of articles at temperatures differing from ambient and to a device for utilization of solid carbon dioxide, which device is usable in conjunction with such containers.
2. Prior Art
For many years frozen CO.sub.2 or "dry ice" has been extensively used in industry in a variety of manufacturing processes and also as a convenient cooling agent used in transport of frozen foods such as ice cream. In the form of dry ice, carbon dioxide provides a good refrigerant as its heat absorbtion upon subliming to gas at minus 109.degree. F. is for every Kg (lb) in weight equivalent to 574 kj (247 BTU). This absorbtion rate is 71% greater than water ice which is only 336 Kj/Kg (144 BTU/lb), and when used, for example, to cool the interior of a portable insulated food or beverage container, the comparatively lesser volume occupied by the dry ice will permit a greater part of the container volume to be used for food or beverage storage. Also, as it sublimates from a solid directly into an inert harmless gas, there is no water damage or drainage problems as involved with water ice, and its very low temperature -109.degree. F., makes practical the portable storage of frozen foods and ice cream for outdoor use.
In the past, whilst dry ice has been freely available to the general public direct from manufacturers, its use, while economically viable as compared with conventional water ice, has been plagued by a number of practical difficulties relating to its distribution and its use by the layman. In both areas its problems relate directly to the very low temperature at which dry ice changes state, so that in order to maintain it in solid form very sophisticated and expensive refrigeration plant is required. This makes automatic vending machines impractical; however, dry ice can be distributed if transported in bulk using very well insulated containers. Its slow sublimation losses can be commercially tolerated. A growing demand by the public for dry ice is foreseen and manufacturers have plans for widespread distribution. In commercial fields, solid CO.sub.2 has been found to provide a convenient and economic refrigerant and various techniques and expertise have been developed to successfully utilize its properties. However, the average citizen, being inexperienced and unaware of its relative super cold nature, may treat it in the same manner as water ice, thus resulting in over cooling of food and drinks, burst glass containers and a comparatively short ice life. Being so cold it demands some respect in handling as it will burn the skin if left in contact for any length of time. If a child is imprudent enough to place a piece in the mouth, as is a common practice in hot weather with conventional ice, serious burns can result. These aspects, and the lack of promotion by the manufacturers, have retarded the general use of solid CO.sub.2.
A most convenient way of producing dry ice is to form it into pellets, as these are easily handled and, unlike crushed water ice, have a dry nature and no tendency to conglomerate. However, such pellets present very large surface areas; while this aspect is desirable for rapid heat extraction for many applications, its rapid absorbtion rate gives a correspondingly shorter ice life. As the pellets start to sublime slowly at -109.degree. F., even frozen food, at say -30.degree. F., will still provide an effective heat sink, and the insulation properties of the container must be substantial to prevent loss to the external environment.
Cold storage containers are alreadly known which can utilize solid CO.sub.2. For example, U.S. Pat. Nos. 1,901,000 to Robe, 2,515,750 to Waller, 2,677,245 to Edmondson and 2,780,074 to Haanan are typical. U.S. Pat. No. 3,896,658 to Hahn also describes a device utilizing solid refrigerant to cool glass plates positioned on a projecting surface of the device. Generally, however, prior devices and containers are not specifically suited to everyday use, being too large and/or costly in structure for this purpose. An object of the invention is to provide a container which is relatively simple in structure and yet efficient in operation.